That depends on how you define "cool" — and that's exactly what the first speaker of the day attempted to do, delving into "why cool matters" as a fitting start to an annual event promoting "ideas worth spreading."

"Cool is the process by which iconic rebels carve out new cultural space for a given generation," Joel Dinerstein, a Tulane University professor, told a crowd of about 1,800 inside the Tennessee Performing Arts Center.

"Cool is not a hero or a saint," he said, sampling from the stories of superstars Miles Davis, Elvis and Missy Elliott. "Cool is wilder and more original."

Dinerstein set the tone — perhaps providing a challenge to the crowd — by elevating what he sees as the key ingredients of world-changing pop culture icons.

They create rebellious new things in response to social needs, he said. And over time, their radical ways of living and thinking become mainstream. He mostly spoke of nonconformity in jazz and rock music, but he also riffed off of Johnny Cash as a man dressed in black and singing for the "poor and beaten down."

The day roamed widely, showcasing avant-garde music, examining the future of radio and taking on topics of utmost seriousness.

Derri Smith, founder of End Slavery Tennessee, forced the crowd to confront the presence of sex trafficking in Nashville. Her nonprofit helps care for survivors, and she used her time to mix statistics and real-world examples of the successes and heart-wrenching failures she's experienced.

While one teen has begun a journey to freedom, another still in treatment was recently found dead from a drug overdose.

"Our world is full of people longing for a community of support," Smith said.

Provided the audience, Smith used her time to urge human kindness in daily interactions.

"When we embrace the importance of being human in all our encounters," she said. "We take a stand in our dehumanized world."

After thunderous applause, George returned to the stage. Minutes earlier, he'd donned a pair of sunglasses to match Dinerstein's talk about cultural rebels. After Smith, he spoke of the passion of the presenters.

"That's the beautiful thing about TEDx," George said, "is you get people from all across the country, different walks of life."